Display tag and categories as tree

[eikek]

From here https://github.com/eikek/docspell/issues/568#issuecomment-761533086 (and following comments)

This is only about the view of the category and tags. Currently the list of categories is rendered below a list of tags, where both are sorted by must usage. Another option is to provide a nested tree-like view.

This is probably also required for nested tags, if #413 is done first, then this issue is void.

[gandy92]

Would this also be possible for the drop-down when adding new tags? It could show the tags grouped by their category like this:

Category 1
   tag 1a
   tag 1b
   tag 1c
Category 2
   tag 2a
   tag 2b

Searching for a tag by typing in the field would reduce the list but keep the grouping, e.g. for search term b:

Category 1
   tag 1b
Category 2
   tag 2b

Not sure if the search should match the category, maybe if it starts with a colon, e.g. for search term :ory 2:

Category 2
   tag 2a
   tag 2b

[eikek]

Hey, sorry missed this message somehow. Yes indeed, the new component should handle both cases. There is also #960 that touches this topic.